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Journal of Asian Ceramic Societies 3 (2015) 206–211

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Physical, thermal, structural and optical absorption studies of vanadyl doped magnesium oxy-chloride bismo-borate glasses M.S. Dahiya a,∗ , S. Khasa a , A. Agarwal b a b

Physics Department, Deenbandhu Chottu Ram University of Science & Technology, Murthal 131039, India Applied Physics Department, Guru Jambheswar University of Science & Technology, Hisar 125001, India

a r t i c l e

i n f o

Article history: Received 4 January 2015 Received in revised form 23 February 2015 Accepted 23 February 2015 Available online 13 March 2015 Keywords: Melt-quenching Optical basicity Glass transition Glass stability Optical band gap

a b s t r a c t Oxy-chloride bismuth-borate glasses with composition xMgCl2 ·(30 − x)MgO·20Bi2 O3 ·50B2 O3 containing 2 mol% doping of V2 O5 (x = 12, 15, 20, 25 and 30) are prepared by melt-quenching technique. The structural, thermal and optical behaviors are explained by analyzing the data obtained from density (D), molar volume (Vm ), theoretical optical basicity (th ), differential scanning calorimetry (DSC), FTIR and UV–vis results. A decrease in D and increase in Vm (except for sample MBV3 for which D is maximum) on increasing chloride content suggests the formation of non-bridging oxygen atoms. Maximum glass transition temperature (Tg ) and crystallization temperature (Tx ) have been observed for sample MBV3. The glass stability (S) and stability ratio (S/Tg ) have been calculated from the values of Tg and Tx and both are having maximum values for sample MBV3. Study of the FTIR spectra in the mid-IR range reveals the presence of both triangular and tetrahedral coordinated boron. The optical studies through UV–vis spectral analysis show non-sharp edge. The optical band gap (Eg ) is also maximum for sample MBV3. © 2015 The Ceramic Society of Japan and the Korean Ceramic Society. Production and hosting by Elsevier B.V. All rights reserved.

1. Introduction Borate glasses have some unique properties such as reduced thermal expansion, resistance to thermal shock, enhanced toughness, strength, chemical resistance and durability which makes them suitable for use in fiberglass [1]. It is known that Bi3+ ion has small field strength so Bi2 O3 cannot form glass by itself but in the presence of B2 O3 glass formation is possible. Heavy metal oxide (HMO) doped glasses have attracted considerable attention due to their high refractive index, high infrared transparency, thermal stability and high density. It is also considered that the addition of Bi2 O3 results in increased stability and chemicals durability of oxide glasses making these glasses suitable for use in ceramics, reflecting windows, etc. [2]. When we add alkali and alkaline earth metal oxides to the bismuth-borate glass matrix it results in network formation or modification. Subsequently it gives rise to

∗ Corresponding author at: INSPIRE Fellow, Department of Physics, DCR University of Science & Technology, Murthal, Sonepat 131039, Haryana, India. Tel.: +91 99969 87095/130 2484136; fax: +91 130 2484003. E-mail address: [email protected] (M.S. Dahiya). Peer review under responsibility of The Ceramic Society of Japan and the Korean Ceramic Society.

different non-linear optical properties making these glasses suitable for optoelectronic applications [3]. The formation of glass in the system ZrF4 -BaF2 -NaF-Nd3 (the so-called halide glass) was studied in 1974 by Poulain and Lucas [4]. The first practical application of BeF2 based halide glass as the glasses having lowest refractive index and highest Abbe number, was reported by Baldwin et al. in 1981 [5]. Besides these properties, the halide glasses are highly toxic and hygroscopic resulting in few studies on these glasses [6]. The oxy-halide glasses are quite important for applications as host materials in high power laser systems [7]. Subsequently, these glasses due to high ionic conductivity of mobile halogen ions can be used in fuel/solar cells [8]. Moreover, the oxy-halide glasses containing phosphates are believed to be less thermally stable [9] but their borate counterparts may be having good thermal stability as observed in our work. Although many reports are available on the electrical [10–17] properties of oxyhalide borate glasses but when it comes to the study of thermal and optical properties relatively lesser attention has been paid [18–20]. As V4+ , vanadium is usually coordinated to six ligands forming an octahedral complex and with oxygen as ligand, one V O bond becomes very distinct which is termed as vanadyl ion (VO2+ ) [21]. The vanadium is generally used as impurity for understanding the orientation, phase transition and structural properties of the host glass and is studied with interest in the recent past [22–24]. In our earlier work, we have studied the oxy-chloride borate glasses in

http://dx.doi.org/10.1016/j.jascer.2015.02.006 2187-0764 © 2015 The Ceramic Society of Japan and the Korean Ceramic Society. Production and hosting by Elsevier B.V. All rights reserved.

M.S. Dahiya et al. / Journal of Asian Ceramic Societies 3 (2015) 206–211

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Table 1 Density (D), molar volume (Vm ), theoretical optical basicity (th ), glass transition temperature (Tg ), crystallization temperature (Tx ) and glass stability (S) and stability ratio (S/Tg ) of xMgCl2 ·(30 − x)MgO·20Bi2 O3 ·50B2 O3 + 2% V2 O5 (x = 12, 15, 20, 25 and 30) glasses. Sample code

x

D (g/cm3 )

Vm (cm3 /mol)

th

Tg (◦ C)

Tx (◦ C)

S (◦ C)

S/Tg

MBV1 MBV2 MBV3 MBV4 MBV5

12 15 20 25 30

3.76 3.51 3.68 3.43 3.40

32.0 35.5 36.0 41.5 44.0

0.465 0.468 0.472 0.476 0.480

523 524 550 524 529

822 791 949 812 810

299 267 399 288 281

0.57 0.51 0.73 0.55 0.53

the systems BaO·BaCl2 ·B2 O3 [25,26] and CaO·CaCl2 ·B2 O3 [27,28] containing vanadyl ions. The oxy-chloride systems are presumed to result in volatilization of chlorine during melting, so the final composition may differ a bit from the actual composition [29]. In the system of oxide glasses containing alkaline earth oxides, the MgO-B2 O3 system is of particular interest because MgO is a principal constituent of the linings of steelmaking furnaces and part of furnace slag consists of MgO [30]. However the MgO-B2 O3 system is characterized by the narrowest glass forming region among the family of alkali earth borate glasses [31]. The addition of Bi2 O3 can result in wider glass forming region and lower melting temperatures, which results in an ease in glass formation. Moreover, the oxy-chloride counterparts in MgO-B2 O3 -Bi2 O3 system may serve as hosts (owing good thermal stability) for high amplification laser systems. Keeping in view the above facts, we have prepared the magnesium oxy-chloride bismo-borate glasses with composition xMgCl2 ·(30 − x)MgO·20Bi2 O3 ·50B2 O3 (x = 12, 15, 20, 25 and 30) and studied their thermal, structural and optical properties. The results along with their interdependence are reported in the present paper.

The DSC measurements of samples in the bulk form were carried out in the temperature range of 200–1000 ◦ C on a thermal analyzer (Perkin Elmer STA 6000). The heating rate and nitrogen flow rate used to carry out analysis were 10 ◦ C/min and 100 ml/min respectively. For FTIR measurements, as-prepared samples were first crushed into fine powder and then mixed in approximately 0.15 g of KBr in the ratio ∼1:100. A circular pellet of diameter 13 mm was formed with this mixture using a hydraulic press. The pellet so formed was analyzed for FTIR using universal sample holder and a Perkin Elmer Frontier FTIR spectrophotometer in the mid-IR range. For any sort of noise and background correction Spectrum 10 software provided with the IR system was used [35]. For optical absorption measurements, the samples with thickness ranging 0.5–1.2 mm were polished to optical quality. The absorption and transmission spectra of the polished samples were recorded in the wavelength range of 200–800 nm at ambient temperature using a UV–vis spectrophotometer (Shimadzu UV2450).

2. Experimental

3. Results and discussion

2.1. Sample preparation

3.1. Density, molar volume and basicity

The preparation was carried out using standard melt-quenching technique. The analar grade chemicals H3 BO3 , MgO, MgCl2 , Bi2 O3 , and V2 O5 required as the starting materials were obtained from Loba Chemie. Each chemical was crushed and weighed in proper amount by using a digital electronic balance (CAS CAUY220). The powders were then mixed with an agate pestle and mortar for half an hour. The mixture so obtained was put into a high alumina crucible for melting at 1100 ◦ C in an electrical muffle furnace for one hour. The melt was then rapidly quenched by sandwiching it between two pre-heated stainless steel plates to obtain samples in the form of discs [32]. Titular representation of the prepared samples is MBV1, MBV2, MBV3, MBV4 and MBV5 for x = 12, 15, 20, 25 and 30 respectively.

The measured values of density (D) and calculated values of molar volume (Vm ) are reported in Table 1. These values are of the same order as reported for alkaline earth oxy-halide glasses [25]. In oxide glasses, study of D and Vm becomes important as both are presumed to provide very good insight about the network forming and modifying units. The variations of Vm and density are shown in Fig. 1. It is quite visible from this figure that Vm is increasing and density is decreasing (except for sample MBV3). The increase

2.3. DSC, FTIR and optical absorption measurements

2.2. Density and basicity measurements The density has been measured using Archimedes principal as D = Dx

Wa Wa − Wx

(1)

where Dx is density of xylene, Wa is weight of sample in air and Wx is weight in xylene. The molar volume (Vm ) thus has been calculated as Vm = M/D with M as the molecular mass of the sample. The theoretical optical basicity is calculated [33] by using the following relation: th =

Zr i i

Zo i

(2)

where Zi is the oxidation number of the cation i, ri is the ratio of cation i with respect to total number of oxides and i = 1.36 (xi − 0.26) with xi as Pauling electro-negativity [34] and Zo as cation oxidation number.

Fig. 1. Composition dependence of density and molar volume of xMgCl2 ·(30 − x)MgO·20Bi2 O3 ·50B2 O3 + 2% V2 O5 (x = 12, 15, 20, 25 and 30) glasses.

208


are also maximum for sample MBV3 which hints that the sample MBV3 is more thermally stable as compared to other compositions. Tg is assumed to depend on the oxygen packing density in the glass networks [44]. The addition of salt to borate glasses is expected to decrease the Tg by creating voids in the glass matrix [45], but contrary to this, we have observed an increase in Tg for sample MBV3. The increase in Tg and thermal stability may be due to the participation of Cl− ions in the network formation [26,10]. Also as predicted by the density and molar volume results, the highest stability observed for MBV3 sample may be due to least number of non-bridging oxygen atoms corresponding to this particular composition. 3.3. FTIR measurements

Fig. 2. DSC curves of xMgCl2 ·(30 − x)MgO·20Bi2 O3 ·50B2 O3 + 2% V2 O5 (x = 12, 15, 20, 25 and 30) glasses.

in Vm may be due to conversion of bridging oxygen atoms into nonbridging oxygen atoms. These results are also supported by the density variations. But a sharp increase in density for MBV3 sample reveals that the structure is having maximum compactness at this composition. This has been supported by an almost unchanged Vm at this composition. The values of theoretical optical basicity (th ) calculated by using Eq. (2) are reported in Table 1. The role of trends in basicity can be best understood by relating it with the oxide ion polarizability (˛2− o , also termed as oxide ion activity [33]) as [36]: th = 1.67(1 − 1/˛2− o ). The relation reflects that the trends in th 2− are similar to ˛2− o . The values of ˛o are calculated through this relation and are reported in Table 1. The increasing trends in th and hence ˛2− o , as reflected by Table 1, hint towards an enhanced ionic character of the glass system with increase in chloride content. This leads to a decrease in oxygen covalency which results in an increase in ␴-bonding between V4+ and the ligands. The enhancement in sigma bonding results in a decrease in the positive charge on V4+ and thus decreases the ␲-bonding between V4+ and the vanadyl oxygen which leads in an increased bond length of vanadyl oxygen thereby improving the octahedral nature of the V4+ O6 complex [32].

The information regarding arrangements of network structural units in glasses can be obtained by studying the infrared spectroscopy as the absorption band vibrations are independent of the vibrations due to other group of atoms [46,47]. The FTIR spectrographs in the spectral range of 400–2000 cm−1 recorded at ambient temperature are shown in Fig. 3. The spectra contain mainly IR absorption bands of the active borate networks [26]. The bands are observed in three regions which are around 600–800, 800–1150 and 1150–1600 cm−1 respectively. The bond-bending vibrations of B O B linkage in three-coordinated borate units give rise to the absorption bands in the region 600–800 cm−1 [48,49]. An absorption band centered at ∼688 cm−1 is observed in this region which may be assigned to bending of B O B linkage in penta-borate (B2 O5 4− ) units [50,51]. This band has almost constant intensity for all compositions except for sample MBV3 for which the intensity is maximum which indicates enhanced structure compactness corresponding to this composition. The absorption region between 800 and 1150 cm−1 contains the stretching vibrations of fourcoordinated boron atoms in different borate groups [52]. In this region, a band is observed near 1000 cm−1 which may be attributed to BO4 tetrahedral vibrations in tri-, tetra- and penta-borate groups [53]. This band gets split into two independent bands near 880 and 1033 cm−1 for sample MBV3. The band centered at ∼880 cm−1 is a characteristic feature of borate arrangement vibrations of BO4 tetrahedra in penta-borate groups and the band centered at 1033 cm−1 is due to BO4 vibrations in tri-borate groups [48]. Moreover, the area of this band is decreased which may be an indication

3.2. Differential scanning calorimetry (DSC) Fig. 2 shows the DSC thermographs of samples in bulk form. All thermographs were recorded at a constant and same heating rate (=10 ◦ C/min) because the characteristics temperatures are reliant on heating rate [37]. From these thermographs, the values of glass transition temperature i.e. Tg (calculated as the x-axis value at half of the endothermic baseline shift) and crystallization temperature i.e. Tx (given by an exothermic peak after the baseline shift) have been obtained and reported in Table 1. Studies have been done to quantify the glass stability (S) and glass forming ability (GFA) by relating these with various characteristic temperatures like Tm (Onset Melting Temperature), Tg , Tx and Tl (Offset Melting Temperature) of oxide glasses [38–40]. The glass stability (S) can be calculated by the width of super-cooled region between glass transition and crystallization as S = Tx − Tg [41,42]. The stability can also be represented by the ratio S/Tg [43]. The values of S and S/Tg thus calculated are reported in Table 1. From Table 1, it is observed that Tg is almost unchanged for all compositions except for sample MBV3 for which it has maximum value. The values Tx , S and S/Tg

Fig. 3. FTIR spectra of xMgCl2 ·(30 − x)MgO·20Bi2 O3 ·50B2 O3 + 2% V2 O5 (x = 12, 15, 20, 25 and 30) glasses.


Fig. 4. Optical absorption (inset: xMgCl2 ·(30 − x)MgO·20Bi2 O3 ·50B2 O3 + 2% V2 O5 glasses.

transmission) plots of (x = 12, 15, 20, 25 and 30)

towards decrease in non-bridging oxygen atoms corresponding to this composition. These results are very well supported by the trends in D, Vm and thermal studies. The third absorption region, which is a characteristic feature of linkage of boron atoms with triborate groups [54], contains an absorption band around 1370 cm−1 . This band is attributed to B O stretching vibrations of trigonal BO3 units in meta-, pyro- and ortho-borates [53]. For sample MBV3, the intensity of this band has increased, indicating an increase in bridging oxygen atoms as depicted by the previously observed band positions. In the region below wave number 600 cm−1 , two very faint bonds are also observed at 460 cm−1 (for samples MBV3 to MBV5) and 520 cm−1 (for samples MBV1, MBV2, MBV4 and MBV5) respectively. The first band which appears at ∼460 cm−1 is due to the Bi O Bi vibrations in BiO6 octahedra [55,56]. The band that appeared at ∼520 cm−1 is assigned to Bi O bending vibrations in the BiO6 units [46,55,57,58].

209

Fig. 5. Tauc plot corresponding to indirectly allowed Mott’s transition in xMgCl2 ·(30 − x)MgO·20Bi2 O3 ·50B2 O3 + 2% V2 O5 (x = 12, 15, 20, 25 and 30) glasses (inset: intercept used to calculate Eopt for sample MBV1).

plots can be used to calculate the optical band gap, given by the intercept of the curve on x-axis where (˛h)r = 0 (represented by the insets of Figs. 5 and 6) and the band tailing parameter (B) given by the slope of linear portion of the Tauc plots. The calculated values of Eopt and B for both transitions are reported in Table 2. The variations in Eopt can be attributed to the structural changes governed in the glass matrix through the substitution of MgCl2 in place of MgO. It can be seen from Table 2 that Eopt is having maximum values for both Mott’s transitions. The formation of NBOs is supposed to contribute in the valence band maximum (VBM) and shifts it a bit upper as the non-bridging orbitals are more energetic than bonding orbitals [62]. This results in the reduction of the band gap. But a decrease in NBOs will signify the formation of BOs which hints towards the fact that for sample MBV3 the number of BOs are maximum and at this composition the Cl− ions are participating in the network formation as also observed from the analysis of structural and thermal studies.

3.4. UV–vis measurements Fig. 4 represents the optical absorption spectra for all prepared compositions recorded at ambient temperature. The non-sharp absorption edge confirms glassy nature of all prepared compositions. Inset of Fig. 4 shows the transmission spectra. The values of cut-off wavelength (cut-off ) are determined from Fig. 4 and are reported in Table 2. The optical absorption co-efficient (˛()) can be calculated from the region near the absorption edge by the relation [59]: ˛() = A/t, where A is absorbance and t is the thickness of the glass sample. A depends upon the intensity of incident (I0 ) and transmitted (It ) radiations as: A = ln(I0 /It ). ˛() is also related to the photon energy (h), optical band gap (Eopt ) and band tailing (B) through the relation [60]: ˛() =

B(h − Eopt ) h

r

(3)

Here r is the index, value of which signifies the type of transition involved in the matter. It can have values 2, 3, 1/2 and 1/3 corresponding to indirect allowed, indirect forbidden, direct allowed and direct forbidden transitions respectively. For amorphous materials, only indirect transitions are valid [61]. Eq. (3) is used to obtain Tauc plots (h vs. (˛h)r ) corresponding to r = 2 and 3. The obtained plots are represented by Figs. 5 and 6 for r = 2 and 3 respectively. The Tauc

Fig. 6. Tauc plot corresponding to indirectly forbidden Mott’s transition in xMgCl2 ·(30 − x)MgO·20Bi2 O3 ·50B2 O3 + 2% V2 O5 (x = 12, 15, 20, 25 and 30) glasses (inset: intercept used to calculate Eopt for sample MBV2).

210


Table 2 Cut-off wavelength (cut-off ), optical band gap (Eopt ), band tailing (B) and Urbach energy (E) for xMgCl2 ·(30 − x)MgO·20Bi2 O3 ·50B2 O3 + 2% V2 O5 (x = 12, 15, 20, 25 and 30) glasses. Sample code

MBV1 MBV2 MBV3 MBV4 MBV5

cut-off (nm)

519 523 507 500 511

B (cm eV)−1/r

Eopt (eV)

E (eV)

r=2

r=3

r=2

r=3

2.35 2.35 2.37 2.35 2.33

2.15 2.14 2.17 2.16 2.13

20.03 22.67 21.12 18.80 21.41

6.66 7.17 6.87 6.44 6.94

0.216 0.222 0.219 0.200 0.218

to both triangular and tetrahedral boron and revealed least nonbridging oxygen corresponding to MBV3. The optical band gap was found to be lying in the range 2.33–2.37 eV and 2.13–2.17 eV for r = 2 and r = 3 respectively. The values of optical band gap were maximum for sample MBV3. It was concluded that sample MBV3 is characterized by least number of NBOs due to the participation of Cl− ions in network formation. Acknowledgements Authors are thankful to UGC New Delhi for providing financial and experimental support under Major Research Project F. No. 40461/2011(SR). Author M.S. Dahiya is thankful to DST New Delhi for providing financial support under INSPIRE Fellowship. References

Fig. 7. Urbach’s plots for xMgCl2 ·(30 − x)MgO·20Bi2 O3 ·50B2 O3 + 2% V2 O5 (x = 12, 15, 20, 25 and 30) glasses (inset: tangent used to calculate slope to obtain E for sample MBV3).

The optical absorption data are used to estimate the band structure and energy band gap in crystalline and amorphous materials. In optical absorption the photon having energy greater than the band gap is absorbed which leaves behind an absorption edge with an exponential increase in ˛(). The increase in ˛() is related to h as [63]: ˛() = ˛0 eh/E

(4)

Here ˛0 is a constant and E is Urbach energy. The Urbach’s plots (i.e. h vs. ln(˛)) are shown in Fig. 7. The Urbach energy can be calculated from the reciprocal of the slope of the Urbach’s plot in the region of lower photon energy (inset Fig. 7). It can yield information about the disorder effects in amorphous or crystalline systems [64]. Also lack of long-range order in amorphous materials is associated with the tailing of density of states [65]. The materials having larger values of E are believed to have greater tendency to convert weak bond into defects. The obtained values of E for the present glass system lie between 0.200 and 0.222 eV. 4. Conclusion Glasses with composition xMgCl2 ·(30 − x)MgO·20Bi2 O3 ·50B2 O3 containing 2 mol% doping of V2 O5 (x = 12, 15, 20, 25 and 30) were successfully prepared by the melt-quench technique. The density was decreasing and molar volume was increasing (except for sample MBV3) with increase in Cl− content. The theoretical optical basicity was increasing depicting an increase in ionic character of the glasses with increase in MgCl2 content. The glass transition temperature and glass stability were having maximum value for sample MBV3. The FTIR spectra contained absorption bands corresponding

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